Civil Engineering Reference
In-Depth Information
The goal, when measuring property is to know the true value of the measure-
ments. It turns out that knowing the true value of a measurement is not always a
simple task. We have seen that the survey foot is used in the majority of jurisdic-
tions in the United States. The survey foot is specified as a certain fraction of a
meter, which has an internationally defined length. In surveying, when we speak
of the true value of a measurement, we are relating the measurement to a known
and verifiable standard. If we are making measurements with a steel tape, the tape
must be calibrated against the standard. If we are making measurements with a
total station with an electronic distance measuring device (EDM), then the EDM
system must be calibrated to the standard. In actuality, all measurement devices
and methods will contain some error. All we can do is to be sure that our measure-
ment instruments are well calibrated and that we are using proven measurement
techniques so that our measurements will as close to their true value as possible.
The goal, when measuring property is to know the true value of the
measurements.
4.18 The Accuracy of Measurements Using GPS
The error of closure we have been discussing up until now is based on traditional
surveying techniques where linear measurements have been used, for example
measurements taken with a tape or total station. These types of measurements
are based on measuring straight lines between fixed points. If we are using GPS,
the measurements are less dependent on the distance between points and more
dependent upon statistical probabilities of the location of a point. These different
methods for expressing uncertainty in measurements and accuracy standards has
made it more difficult to compare the accuracy and uncertainty of points estab-
lished using linear techniques with those established using GPS.
If we are using GPS, the measurements are less dependent on the distance
between points and more dependent upon statistical probabilities of the loca-
tion of a point.
When working with GPS locations, the horizontal accuracy is expressed using a
positional uncertainty called the
Circle of Uncertainty
. The accuracy of a particu-
lar point is based on the 95 % probability of the point being within a circle having
a specific radius.
1
An example may help to clarify this. Assume we are using a
1
Based on a 95 % confidence level. See Standards and Guidelines for Land Surveying
using Global Positioning System Methods, August 6, 2004, The Survey Advisory Board for
Washington State Department of Natural Resources.
